The majority of countries have mobile radio systems. In order to enable the system operators to satisfy capacity expectations with the frequency bands allotted to the Operator, the mobile radio system is divided into geographical areas that are called cells. The cells may have a radio base station placed in the centre of the cell, wherein a mobile terminal communicates with other mobile terminals and with a land-based telephone network through said radio base station. The cells are designated omnicells in this latter case. One and the same radio base station can cover several cells with the aid of directional antennas, these cells then being designated sector cells.
Radio channels that are allotted to the mobile radio system are shared by those cells that are included in the system. A set of mutually the same radio channels can be used in several cells. Cells that use the same radio channels are spaced sufficiently apart so as not to interfere with one another. This demands careful cell planning.
First-time planning of a cell involves choosing the respective positions of radio base stations and antennas, among other things. The power at which the antennas are permitted to transmit is governed by the positions of the radio base stations and the antennas, which in turn governs the area covered by the base stations. Improvements in capacity, for instance, necessitates the cells being made smaller and therewith the maximum permitted output power to be reduced.
In order to plan a cell accurately and, at the same time, provide the highest possible output power without exceeding a permitted power level, it is necessary to control the power output of the antenna both accurately and quickly.
EP 0684707 A1 teaches an arrangement and a method for controlling the transmission power of radio frequency signals. A power-sensitive element placed within an antenna element enables the transmission power of radio frequency signals to be monitored. The signal power measured by the power sensitive element is converted analogue/digital and delivered to a microprocessor. The microprocessor generates an output signal in relation to said input signal, said output signal controlling a variable attenuator included in the transmitter chain of the arrangement and thus influencing the output power of radio frequency signals by virtue of different attenuation levels of the attenuator. The power control is carried out with each time slot and the power of a transmitted radio frequency signal is not changed during a time slot but is adjusted between time slots.
EP 0695031 A2 teaches a mobile communications arrangement that includes a control circuit for adjusting, or setting, the output power of radio frequency signals. A reference voltage generator generates a reference voltage that is compared with a detected voltage by means of a comparator. The voltage detected corresponds to a power of a radio frequency signal to be transmitted. The detected voltage is obtained by measuring the power of the radio frequency signal with an output power sensor and converting the measured power to a corresponding detected voltage with a detector. The reference voltage corresponds to a desired output power of the radio frequency signal. The comparator generates a difference signal, i.e. a signal corresponding to the difference between the measured power and the reference power, which controls a power amplifier in the transmitter link of the arrangement. The control circuit sets the amplification factor of the power amplifier so that the output power of the radio frequency signal will correspond to the desired output power. Generated reference voltages derive from measurements of received radio frequency signals and the reference voltages are thus dependent on the received radio frequency signals.
Described in JP 07250020 A is a regulator intended for controlling output powers of radio frequency signals that shall be transmitted from a portable telephone or like apparatus. According to this solution, when measuring the power at which radio frequency signals are transmitted, there is used an antenna which receives a part of those radio frequency signals that are sent by the portable telephone. Received radio frequency signals are detected and compared with a d.c. voltage in a comparator and the difference indicating signal generated thereby controls indirectly a variable amplifier in the transmitter link of the portable telephone.